It depends on what your dog is sensitive to. Usually the most common things for sensitive dogs to avoid would be most grains, and then sometimes even potatoes or chicken. Try something grain free and without chicken. If he does well on that, add either chicken or grain back in and see if it affects him. There are lots of other things dogs can be intolerant to, from tomatoes to fish! See if your vet can do an allergy test, or try an elimination diet.

Also try adding some coconut oil for an extra skin-boost. 1tsp per 10lb of bodyweight, but start with only about 1/4 the amount, gradually building up.

Many people do home made diets for sensitive dogs. The Honest Kitchen is one of the favorites around here I know.